1. Field of the Invention
This invention relates to a mold used to manufacture an optical element formed of glass such as a lens or a prism by press-molding of a glass material, a method of manufacturing said mold, and a molding method using said mold.
2. Related Background Art
The technique of manufacturing a lens by press-molding of a glass material without requiring a polishing process has eliminated the complicated steps heretofore required in the manufacture of a lens and has made it possible to make a lens simply and inexpensively, and has recently been used in the manufacture of not only a lens but also other optical elements formed of glass such as prisms.
As the characteristics required of a mold used for the press-molding of such an optical element of glass, mention may be made of the excellence in hardness, heat resisting property, parting property, mirror surface workability, etc. Heretofore, numerous materials such as metals, ceramics and materials coated therewith have been proposed as mold of this type. Mentioning some examples, Japanese Laid-Open Patent Application No. 49-51112 proposes 13 Cr martensite steel, Japanese Laid-Open Patent Application No. 52-45613 proposes SiC and Si.sub.3 N.sub.4, and Japanese Laid-Open Patent Application No. 60-246230 proposed a super-hard alloy coated with a precious metal.
Also, a method using a mold material consisting of carbon is proposed in Japanese Laid-Open Patent Application No. 49-81419, and methods using a mold material coated with carbon are proposed in Japanese Patent Publication No. 55-11624, Japanese Laid-Open Patent Application No. 63-203222, Japanese Laid-Open Patent Application No. 61-183134, Japanese Laid-Open Patent Application No. 61-281030 and Japanese Laid-Open Patent Application No. 64-83529.
However 13 Cr martensite steel suffers from the disadvantages that it is ready to be oxidized and that Fe is diffused in glass at a high temperature to thereby color the glass. SiC and Si.sub.3 N.sub.4 are generally regarded as being difficult to be oxidized, but still suffers from the disadvantage that it is also oxidized at a high temperature and a film of SiO.sub.2 is formed on the surface thereof and therefore fusion with glass is caused and further the workability of the mold itself is very bad due to the high hardness. The material coated with a precious metal is difficult to be fused, but it is very soft, and this leads to the disadvantage that it is ready to be injured and also ready to be deformed.
Also, the glassy carbon shown in Japanese Laid-Open Patent Application No. 49-81419 is weak in structural strength and therefore suffers from the disadvantage that during press molding, the shape of the mold surface is distorted or the deterioration of the surface roughness by flaws or the like occurs.
On the other hand, a mold coated with a carbon film has been proposed, whereas the carbon film is not limited to one kind, but from the viewpoint of crystal structure, it can be divided broadly into: (i) a diamond polycrystalline film; (ii) a graphite film or a glassy carbon film having a crystalline property; (iii) a diamond-like carbon film comprising a diamond crystallite phase and an amorphous phase; and (iv) a carbon film of high hardness composed of amorphous or microcrystal (an aggregate of crystallites) comprising SP.sup.2 and SP.sup.3 -hybrided carbon.
The diamond polycrystal film mentioned under item (i) above is high in surface hardness and free of fusion with molded glass as well as low in reactiveness, but has the disadvantage that due to its being a polycrystalline film, it is inferior in surface roughness and the process of polishing the mold is difficult. The graphite film or the glassy carbon film mentioned under item (ii) above is low in hardness and structural strength and is inferior in the anti-oxidation property at high temperatures and the surface accuracy or the surface roughness is deteriorated, and this is not preferable.
Also, the diamond-like carbon film disclosed in Japanese Laid-Open Patent Application No. 63-203222 is made into a film by the use of the plasma chemical vapor deposition (=PCVD) method under a condition of relatively high substrate temperature, and is the diamond-like carbon film of item (iii) containing a diamond crystal phase. Therefore, the film is of inhomogeneous quality and it is difficult to obtain a smooth film which is high in the surface accuracy. Further, in the repetitive molding process, the mold surface is oxidized little by little, but the film of inhomogeneous quality is not generally uniformly oxidized and therefore has the disadvantage that the deterioration of the surface roughness is keen. In addition, the crystallization of the film is ready to progress due to molding at high temperature, and this leads to the disadvantage that the quality of the film is deteriorated and the hardness and adhesion are deteriorated.
A method using a diamond-like carbon film is disclosed in Japanese Laid-Open Patent Application No. 61-183134, but an amorphous film of this kind comprises SP.sup.2 carbon and SP.sup.3 carbon, and is difficult to clearly distinguish from a glassy carbon film or an amorphous carbon film chiefly comprising SP.sup.2 carbon. So, hereinafter, as regards the carbon film of high hardness composed of amorphous or micro crystal comprising SP.sup.2 and SP.sup.3 -hybrided carbon, a film containing a small amount of hydrogen in its composition will be referred to as a hard carbon film, and a film containing hydrogen to a certain concentration or greater will be referred to as a hydrogenated amorphous carbon film (a-C:H film).
The hard carbon film disclosed in Japanese Laid-Open Patent Application No. 1-83529 is sputtered at a relatively low substrate temperature and therefore is presumed to be an aggregate of diamond-like and graphite-like hyperfine particles, and is considered to be classified into the hard carbon film of item (iv) containing amorphous or microcrystal.
This film, when made, has no hydrogen atom contained therein and therefore is high in the homogeneity of film, but has the tendency that the better the surface smoothness, the greater the film become stress. This leads to the disadvantage that in the molding process, due to the release of the stress, the film is ready to peel off in a minute area or generally with the polishing flaw of the mold base material, the remaining polishing agent, dust, or the grain boundary or the partiality of composition of the mold base material as an occasion.
In addition, this film is not terminated by hydrogen atoms and therefore, much dangling bond is ready to be contained therein, and this leads to the disadvantage that the film reacts to glass and is fused thereto or causes a component in the glass to be reduced and deposited on the surface of the glass to thereby reduce the optical characteristic or the readiness with which it reacts to the oxygen in the atmosphere is liable to reduce the hardness and the adhesion by the deterioration resulting from oxidation.